1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device that is adaptive for increasing display quality by preventing flickers and DC image sticking, and a driving method thereof.
2. Discussion of the Related Art
A liquid crystal display device controls the light transmittance of liquid crystal cells in accordance with video signals, thereby displaying a picture. An active matrix type liquid crystal display device actively controls the displayed images by switching data voltages supplied to a thin film transistor TFT formed at each liquid crystal cell Clc, as shown in FIG. 1, thus increasing the display quality of motion pictures. As shown in FIG. 1, a reference numeral “Cst” represents a storage capacitor for keeping data voltages charged in the liquid crystal cell Clc. “DL” represents a data line to which the data voltages are supplied, and “GL” represents a gate line to which scan voltages are supplied to activate the thin film transistor TFT.
The liquid crystal display device is driven by an inversion method where polarities are inverted between adjacent liquid crystal cells and between successive frame periods, in order to reduce the deterioration of liquid crystals and to decrease DC offset components. If any one polarity between two polarities of the data voltage is dominantly supplied for a long time, a residual image is generated. Such a residual image, referred to as “DC image sticking,” is created because a voltage of the same polarity is repeatedly charged in the liquid crystal cell.
An example of when DC image sticking occurs is when interlaced data voltages are supplied to the liquid crystal display device. An interlace method applies odd-numbered line data voltages to liquid crystal cells in odd-numbered horizontal lines during odd-numbered frame periods and even-numbered line data voltages to liquid crystal cells in even-numbered horizontal line during even-numbered frame periods.
FIG. 2 illustrates a waveform diagram representing an example of data voltages supplied to a liquid crystal cell Clc using an interlace method. The data voltages of FIG. 2 represent data voltages supplied to any one of the liquid crystal cells disposed on an odd-numbered horizontal line.
As shown in FIG. 2, using the interlace method, high data voltages (i.e., image data) are supplied to a liquid crystal cell Clc (not shown) disposed on an odd-numbered horizontal lines only during odd-numbered frame periods. In addition, because the polarity of the data voltages alternate every frame period, the liquid crystal cell Clc is supplied with high voltages that are positive only during odd-numbered frame periods and with low voltages (i.e., no image data) during even-numbered frame periods. Because of this, the positive data voltage, like the waveform shown in the box of FIG. 2, becomes more dominant than the negative data voltage over a four-frame period, for example, thus creating a DC image sticking phenomenon.
FIG. 3 shows exemplary images of an experimental result of a DC image sticking phenomenon generated due to interlace data. For example, if an original picture (e.g., left image of FIG. 3) is displayed on a liquid crystal display panel using the interlace method for a fixed period of time, a DC image sticking pattern of the original picture (e.g., right image of FIG. 3) dimly appears when a data voltage of an intermediate gray level (e.g., gray level of 127) is supplied to all of the liquid crystal cells Clc of the liquid crystal display panel after the original picture.
As another example of when the DC image sticking occurs is when an image is moved or scrolled at a fixed speed because the image data voltage of the same polarity is repeatedly accumulated in the liquid crystal cell Clc based on the scroll speed (or moving speed) and the size of a picture which is scrolled (or moved). FIG. 4 shows exemplary images of an experimental result of a DC image sticking phenomenon generated when moving an oblique line or character pattern at a fixed speed.
In a liquid crystal display device, the display quality of motion pictures is degraded not only because of the DC image sticking, but also because of a flicker phenomenon caused by a visual perception of difference in brightness. Accordingly, in order to improve the display quality of a liquid crystal display device, the DC image sticking phenomenon and the flicker phenomenon need to be prevented or minimized.